CHAPTER I 



The Life of Protozoa 



History and General Considerations. Anatomy and 

 Physiology. Characteristics of the Cysts. Life History. 

 Specificity of Parasite for Host. Resistance and Immunity 

 of Hosts. Encystment. Reproduction. 



HISTORY AND GENERAL CONSIDERATIONS 

 OF PROTOZOA 



The discovery that our universe is populated by 

 living organisms of insufficient size to be seen by the 

 unaided human eye belongs to Antoni van Leeuwen- 

 hoek (1632-1723), the great Dutch microscopist. He 

 it was in 1675 who first saw, with the aid of a micro- 

 scope of crude proportions as compared to the appara- 

 tus of the present day, minute animals which are 

 called Protozoa. He studied them as free living 

 forms, as parasites of lower animals, and as parasites 

 of man, finding himself the host of one, which Dobell 

 identifies as Giardia (Lamblia) i7itestinalis. 



He thus became the discoverer of a world of life, 

 unknown before that time and now unknown to the 

 vast majority of people, the population of which far 

 exceeds in number the visible animal population of 

 the world. 



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